After the beheading of another Western captive by ISIS, an international conference convened Monday in Paris to talk about how to tackle the threat posed by the Islamic extremist group.

The Sunni jihadist group underlined its barbaric credentials over the weekend, posting a video showing the beheading of British aid worker David Haines and threatening the life of another hostage from the United Kingdom.

It was the third videotaped killing of a Western hostage released in less than a month.

The latest killing, ISIS said, was "a message to the allies of America" - a direct challenge to the United States.

President Barack Obama announced last week that the United States would lead "a broad coalition to roll back this terrorist threat," and that U.S. airstrikes against ISIS would expand from Iraq into Syria.

The United States has said nearly 40 nations have agreed to contribute to the fight against ISIS, which has seized control of large areas of northern Iraq and Syria. But it remains unclear exactly which countries are on that list and what roles they'll play.

The Paris conference Monday is being hosted by French President Francois Hollande and his Iraqi counterpart Fuad Masum.

At the start of the event, Hollande said there was "no time to lose" in international efforts against ISIS.

Ukrainian border guards are in Russia and have started to examine the contents of a huge convoy of trucks that Moscow says is carrying relief goods for civilians in war-torn eastern Ukraine, a border guard official said Friday.

The Ukrainian government has expressed fears that the mission is a camouflaged effort to smuggle supplies to pro-Russian rebels and has vowed to keep the convoy out.

Russia insists that it should be permitted to send aid to the conflict-battered region, many of whose residents are Russian speakers.

The Ukrainian government, whose forces have been fighting the pro-Russian rebel groups for months, has said any aid needs its approval and has to go through the Red Cross or the United Nations.

Serhay Astahov, a spokesman for the the Ukrainian border guard service, told CNN on Friday that border guards had started inspecting the trucks at a checkpoint on the Russian side of the border from Ukraine's Luhansk region.

Israel reported rocket fire from Gaza on Friday as a three-day cease-fire in the region came to an end without an agreement to extend it.

More than 18 rockets were fired at Israel after the 72-hour cease-fire expired at 8 a.m. Friday (1 a.m. ET), the Israel Defense Forces said. Two of the rockets were intercepted, 14 hit open areas and two came down in Gaza, the IDF said.

Hamas, the Islamic militant group that holds power in Gaza, said that Palestinian officials at talks in Cairo hadn't agreed to extend the truce but would continue negotiations.

But Israeli government spokesman Mark Regev told CNN that the resumption of rocket fire means Hamas has "broken the fundamental premise of the talks in Cairo." Israel had said Thursday it was willing to extend the truce unconditionally.

Two militant factions who have fought alongside Hamas in Gaza, Islamic Jihad and the Al-Nasser Salah al-Din Brigades, said they had fired rockets at Israel on Friday.

It wasn't immediately clear how the Israeli military would respond to the rocket fire. The IDF pulled its ground forces out of Gaza on Tuesday but said they were maintaining "defensive positions" around the territory.

Roughly three hours before the truce was due to end, the IDF said two rockets fired from Gaza had hit southern Israel, without causing any casualties. "Terrorists have violated the cease-fire," the IDF wrote on Twitter.

It was unclear who in Gaza, where multiple militant factions are active, launched the two rockets, which landed near Eshkol in southern Israel.

Hamas denies responsibility for the rockets fired before the cease-fire ended, said Sami Abu Zuhri, a spokesman for the group. The allegations "are based on Israeli reports aimed at confusing the situation," the Gaza-based spokesman said.

Mangled limbs wrapped in blood-stained rags were strewn across rooms in the school.

Run by the United Nations, the building was meant to be a refuge for more than 3,000 civilians fleeing the deadly conflict raging between Israel and Hamas in Gaza.

But shells hit it in the early morning hours as those inside slept, punching huge holes in the roof and killing at least 20 people, according to the Palestinian Health Ministry.

"We saw the shells when they hit and shrapnel was falling like rain," said Sanaa abu Gerard, a woman who witnessed the blasts. "I was so scared and the school filled with smoke. We poured water in our eyes just to see."

The deadly strike Wednesday underscores growing concerns that in Gaza, where fighting is taking place in densely populated areas, safe havens appear to be anything but safe.

It's the sixth time a facility run by the United Nations Relief and Works Agency (UNRWA) has been struck in the current conflict, intensifying questions about the tactics being used by both sides in areas crowded with civilians.

"People who go to these places expect that they go there because they will be safe," said Pierre Krahenbuhl, the commissioner-general for UNRWA. "And here is the confirmation that it appears that there is nowhere where you can be safe."

The United Nations said it thinks the rounds that hit the school, in the Jabalya refugee camp in northern Gaza, were fired by Israeli artillery. The Israeli military said it is reviewing what happened, acknowledging that it exchanged fire with militants in the area.

Fierce fighting between Israeli troops and Hamas militants continued near Gaza City on Monday as the death toll from the conflict rose above 500.

Heavy bombardment hit areas east of the city, particularly the neighborhood of Shaja'ia, where a large Israeli assault Sunday contributed to the deadliest day of the war so far.

As clashes escalated over the weekend, Hamas said it had captured an Israeli soldier - a claim Israel later denied.

With no sign of either side backing down in a conflict in which most of the victims have been civilians, U.S. Secretary of State John Kerry is due to arrive in Egypt on Monday to push for a cease-fire.

Speaking to CNN's "State of the Union" on Sunday Kerry said that the United States supports Egypt's initiative for a truce and "will work for a fair cease-fire."

The United States has "shown our willingness to try to deal with the underlying issues," but Hamas "must step up and show a level of reasonableness," he said.

"No country, no human being, is comfortable with children being killed, with people being killed, but we're not comfortable with Israeli soldiers being killed either or with people being rocketed in Israel," Kerry said.

Deadly clashes broke out after Israeli tanks drove into Gaza and launched a ground operation that escalates the conflict with Hamas.

The incursion Thursday night follows 10 days of Israeli bombardment of Gaza that has killed more than 250 people. Israel launched the aerial offensive last week, saying it aimed to halt the firing of Hamas rockets from Gaza into Israel.

At least three mortars were fired from Gaza toward southern Israel on Thursday morning, some two hours after a temporary lull in hostilities went into effect to allow humanitarian supplies into the area.

The mortars fell in open areas and no injuries were immediately reported, the Israeli military said. Mortars are smaller and have a more limited range than rockets.

Israel, which said it would honor the cease-fire but not sit idle if attacked, did not immediately respond.

The five-hour long temporary halt in fighting went into effect at 3 a.m. ET, requested by the United Nations to offer a brief respite in a conflict that has killed more than 220 people.

Banks opened for the first time in 10 days in Gaza and residents poured into the streets.

Red Cross officials visited hospitals and damaged houses to assess medical needs, and worked with local officials to quickly fix water pipelines, which has left hundreds of thousands without water.

There were no reported air strikes since the cease-fire began. Still, the fear of death hung heavily over Gaza. The health ministry warned civilians to avoid gathering in squares.

"Should the humanitarian window be exploited by Hamas or other terror organizations for the purpose of launching attacks against Israeli civilian or military targets the IDF will respond firmly and decisively," the Israeli military said.

The Israeli Defense Forces said Wednesday that it warned residents of three areas in Gaza to vacate their homes because of its plans to carry out airstrikes on Hamas and other terrorist groups.

The IDF said it used recorded messages, text messages and leaflets to alert the populations in Beit Lahyia, Shuja'iya and Zeitoun, places where it said "a high volume of rocket fire at Israel has originated."

"We are all scared but we are force to live in this and there is nothing for us to do," said Abu Musbah, a, 21-year-old member of Islamic Jihad, one of the groups that is firing rockets at Israel. "The children are scared but we struggle to continue our lives."

He said he had already evacuated all of his family members from his house in Shuja'iya.

A woman in the southern Gaza town of Khan Yunis said some people had fled there from harder hit areas in the north and east of the territory, filling some homes with multiple families.

"Those who have family here have come," said the woman, who asked not to be identified for fear of repercussions from talking to the news media.

She said she believed other people were staying in their houses, regardless of warnings from lsraeli leaflets or other means.